Unloading confining pressure tests with fixed displacement after conventional triaxial compression are conducted. In this course, testing machine does not do any works for specimens. And specimens' failures are caused by deformation energy of themselves. Based on the analysis of failure feature, deformation, strength and unloading ratio, the relationship between rock failure under unloading condition and rockburst is discussed. The results show that rock failure under unaxial compression is similar to light rockburst. With the increase of confining pressure, rock failure phenomena is tend to change from middle rockburst to violent rockburst. As for the rock under three-dimensional stress, the decrease of stress for a certain direction could possibly lead to rock failure in low stress state, Meanwhile, the elastic strain energy releases which could result in rockburst. During the excavation of underground engineering, rockburst can be controlled by the progress of construction.
Rock usually bears the effect of three-dimensional stress under complicated stress state. The excavation of underground engineering caused the release of rock stress in one dimension, which destroyed the original mechanical balance and led to the re-distribution of rock stress and even the failure of rock. Rockburst is a typical unloading fracture phenomena in the course of excavation under high stress level condition. In the past, the mechanical tests research of rockburst usually adopted loading tests, which is not conformed to real stress path of rockburst. In fact, unloading triaxial tests is more suitable to rockburst mechanical problem. With the deep-going development of rock mechanics and the demand of practical engineering, rock unloading tests is gradually conducted [1–8].
During the excavation of underground engineering, the lateral stress along the surrounding of hole is released, and the stress will re-distribute. When the new stress balance state reaches to its limit state, rock failure happened. In excavating of underground engineering at someplace where the ground stress is high, there may be some badly-connected plane fractures which divide the whole surrounding rock into parts. The fractures' development may result in the sudden collapse of tunnel and rockburst.
Assuming the radial stress σr of surrounding rock is the minimum principal stress σ3 and the tangential stress σθ is the maximal principal stress σ1, the radial stress decreases gradually with approaching the free surface in the course of excavating of underground engineering. However, tangential stress increases with approaching the surface at some parts while decreases at some other parts. So, according to the actual excavating process, test plans of triaxial unloading confining pressure are designed. Then, the change of stress state can be modeled and test parameters under different constructing condition are obtained.
In summary, there are two main methods to conduct the triaxial tests of rock unloading confining pressure, that is, the stress control method and the displacement control method.
Stress control method includes two methods, that is, increasing principal stress difference (σ l – σ 3)